Stretch wrapping is a commonly used method of protecting palletized loads of material for shipping. Described generally, stretch wrapping involves wrapping a specialized film around a stack of items such as cases that have been arranged on a pallet. The film is wrapped around the cases under tension and thereby stabilizes the stack to minimize the risk of damage during shipping. Tension can be provided by the memory recovery of pre-stretched film, and tension may also be created by resistance between the load and film dispenser or a combination of the two.
There are many styles and designs of automated or semi-automated stretch wrapping machines, many of which work in cooperation with automated palletizing machines that build the palletized loads. The stretch wrapping machines provide relative rotation between the palletized load and a dispenser that holds a roll of stretch wrapping film. Typically, either the pallet and load are stationary with the dispenser rotating around the load, or the pallet and load are rotated relative to a stationary dispenser. Either way, the stretch wrapping film is wrapped helically up and down the load under tension to stabilize it.
Stretch wrapping machines are used in highly automated production and packaging lines and must be able to keep up with throughput rates of the other equipment used in the palletizing operation so that the stretch wrapping operation does not slow the overall production. As such, the devices often operate at relatively high production rates themselves. But stretch wrapping is not always a simple operation. For example, it is known that with a rectangular load on a pallet the demand for the stretch wrapping film varies as the corners of the load pass by the film dispensing point: the payout demand for film increases as the corner of the load passes the dispensing point and decreases as the film is being dispensed across the side of the load between corners. In addition to the payout rate, the amount of tension on the film has a direct impact on the stability of the load when completely wrapped. Many stretch wrapping machines use tensioning devices to control the tension on the film. However, tension forces vary with rotational position and as a result, proper tensioning is often difficult to maintain with high throughput rates. When the film breaks for any number of reasons (including excessive tension), the operation of the stretch wrapping device is stopped or slowed while the film is reattached to the load, either automatically or with operator intervention.
There is an ongoing need for improved stretch wrapping devices that balance the needs and challenges of keeping up with production rates while wrapping loads with proper film tension to correctly stabilize the loads.
The present invention comprises an improved apparatus and method for automated stretch wrapping of a palletized load.